Add instructions for creating/updating a lint baseline

  `defaults` field, e.g. `platform_service_defaults`, you can add the `lint` property to that common

  module instead of adding it in every module.

## Create or update a baseline

Baseline files can be used to silence known errors (and warnings) that are deemed to be safe. When

there is a lint-baseline.xml file in the root folder of the java library, soong will

automatically use it. You can override the file using lint properties too.

```

java_library {

    lint: {

        baseline_filename: "my-baseline.xml", // default: lint-baseline.xml;

    }

}

```

When using soong to create a lint report (as described above), it also creates a reference

baseline file. This contains all lint errors and warnings that were found. So the next time

you run lint, if you use this baseline file, there should be 0 findings.

After the previous invocation, you can find the baseline here:

```

out/soong/.intermediates/frameworks/base/services/autofill/services.autofill/android_common/lint/lint-baseline.xml

```

As noted above, this baseline file contains warnings too, which might be undesirable. For example,

CI tools might surface these warnings in code reviews. In order to create this file without

warnings, we need to pass another flag to lint: `--nowarn`. The easiest way to do this is to

locally change the soong code in

[lint.go](http://cs/aosp-master/build/soong/java/lint.go;l=451;rcl=2e778d5bc4a8d1d77b4f4a3029a4a254ad57db75)

adding `cmd.Flag("--nowarn")` and running lint again.

## Documentation

- [Android Lint Docs](https://googlesamples.github.io/android-custom-lint-rules/)